Improve AV1 support.

- Require presence of CodecPrivate in AV1 tracks in mkvmuxer and
  mkvmuxer_sample.
- In mkvmuxer_sample, copy CodecPrivate for Video Tracks when
  present.

Change-Id: Ic8e48f8c8b934e543518da19cf17d71b368c30d3
2 files changed